The tetraphenylphosphonium, bis(triphenylphosphoranylidene)ammonium, and n-butylammonium salts of tetrachlorodioxoosmium were prepared. Alkylation of (PPh$\sb4$) (Os(O)$\sb2$Cl$\sb4$) with trimethylsilylmethylmagnesium chloride yields oxotetrakis(trimethylsilylmethyl)osmium. The crystal structure of Os(O)(CH$\sb2$SiMe$\sb3$)$\sb4$ was determined. The physical properties and reaction chemistry of isoelectronic alkyl complexes of osmium(VI), n-butylammonium nitridotetrakis-(trimethylsilylmethyl)osmium, methylimidotetrakis(trimethylsilylmethyl)osmium, and Os(O)(CH$\sb2$SiMe$\sb3$)$\sb4$ are compared. The nitrido complex does not react with carbon monoxide while both the methyl imide and oxo compounds are reductively carbonylated under CO pressure. The methylimido complex reacts to produce the acyl derivatives, Os(N)(C(O)CH$\sb2$SiMe$\sb3$)(CH$\sb2$SiMe$\sb3$)$\sb3$ and Os(N)(C(O)CH$\sb2$SiMe$\sb3$)$\sb2$(CH$\sb2$SiMe$\sb3$)$\sb2$. A mono insertion produce is produced in reaction of the methylimide with tert-butylisonitrile. The nitrido complex forms Lewis acid adducts, is alkylated by organic electrophiles, and reacts with mineral acids. Electrophilic cleavage is slower in the methylimido complex while the oxo complex is stable towards electrophiles. The observed differences in reactivity can be ascribed to subtle changes in bonding between the metan and the terminal heteroatom ligand, and to the formation of a structural isomer of the methylimido complex. Pentahaptocyclopentadienylnitridobis(trimethylsilylmethyl)osmium and pentamethylcyclopentadienylnitridobis(trimethylsilylmethyl)osmium can be made from sodium cyclopentadienide or lithium pentamethylcyclopentadienide and n-butylammonium dichlorobis(trimethylsilylmethyl)osmium. The addition of silver tetrafluoroborate to ($\eta\sp5$-C$\sb5$H$\sb5$)Os(N)(CH$\sb2$SiMe$\sb3$)$\sb2$ formed a silver dimer, $\{$ (($\eta\sp5$-C$\sb5$H$\sb5$)Os($\mu$-N)(CH$\sb2$SiMe$\sb3$)$\sb2$) $\sb2$Ag$\}$(BF$\sb4$). The compound reacted with boron trifluoride to form an adduct in solution and with methyltrifluoromethane sulfonate to form the methylimide, (($\eta\sp5$-C$\sb5$H$\sb5$)Os(NMe)(CH$\sb2$SiMe$\sb3$)$\sb2$) (OSO$\sb2$CF$\sb3$). The crystal structure of the silver dimer was determined. The phosphine complex, Os(N)Cl(CH$\sb2$SiMe$\sb3$)$\sb2$(PMe$\sb3$)$\sb2$, was prepared from (N-n-Bu$\sb4$) (Os(N)Cl$\sb2$(CH$\sb2$SiMe$\sb3$)$\sb2$) and PMe$\sb3$. The salt, (Os(N)(CH$\sb2$SiMe$\sb3$)$\sb2$(PMe$\sb3$)$\sb2$) (BF$\sb4$), was prepared by addition of AgBF$\sb4$ to the chloride compound. NMR and conductivity studies determined that the chloride compound was neutral while the BF$\sb4$ product was cationic. The addition of n-butyl lithium to Os(N)Cl(CH$\sb2$SiMe$\sb3$)$\sb2$(PMe$\sb3$)$\sb2$ formed n-butylnitrido-bis(trimethylsilylmethyl)osmium.